•Chap. VI. Transitions of Organic Beings. 141 



had come to a high stage of perfection, so as to have given them a 

 decided advantage over other animals in the battle for life. Hence 

 the chance of discovering species with transitional grades of struc- 

 ture in a fossil condition will always be legs, from their having 

 existed in lesser numbers, than in the case of species with fully 

 developed structures. 



I will now give two or three instances both of diversified and of 

 changed habits in the individuals of the same species. In either 

 case it would be easy for natural selection to adapt the structure 

 of the animal to its changed habits, or exclusively to one of its 

 several habits. It is, however, difficult to decide, and immaterial 

 for us, whether habits generally change first and structure after- 

 wards ; or whether slight modifications of structure lead to changed 

 habits ; both probably often occurring almost simultaneously. Of 

 cases of changed habits it will suffice merely to allude to that of the 

 many British insects which now feed on exotic plants, or exclu- 

 sively on artificial substances. Of diversified habits innumerable 

 instances could be given : I have often watched a tyrant flycatcher 

 (Saurophagus sulphuratus) in South America, hovering over one 

 spot and then proceeding to another, like a kestrel, and at other 

 times standing stationary on the margin of water, and then dashing 

 into it like a kingfisher at a fish. In our own country the larger 

 titmouse (Parus major) may be seen climbing branches, almost like 

 a, creeper ; it sometimes, like a shrike, kills small birds by blows 

 on the head ; and I have many times seen and heard it hammering 

 the seeds of the yew on a branch, and thus breaking them like a 

 nuthatch. In North America the black bear was seen by Hearne 

 swimming for hours with widely open mouth, thus catching, almost 

 like a whale, insects in the water. 



As we sometimes see individuals following habits different from 

 those proper to their species and to the other species of the same 

 genus, we might expect that such individuals would occasionally 

 give rise to new species, having anomalous habits, and with their 

 structure either slightly or considerably modified from that of 

 their type. And such instances occur in nature. Can a more 

 striking instance of adaptation be given than that of a woodpecker 

 for climbing trees and seizing insects in the chinks of the bark ? Yet 

 in North America there are woodpeckers which feed largely on fruit, 

 and others with elongated wings which chase insects on the wing. 

 On the plains of La Plata, where hardly a tree grows, there is a 

 woodpecker (Colaptes campestris) which has two toes before and 

 two behind, a long pointed tongue, pointed tail-feathers, sufficiently 

 stiff to support the bird in a vertical position on a post, but not so 



